Some CO2 laser material processing applications, such as glass or thin film cutting, require power output variations from the CO2laser to be about ±2% or less at output powers ranging from 100 Watts (W) to 1000 W. The laser discharge is typically driven by a radio-frequency power supply (RFPS). In most of these applications, the laser is operated in a pulsed mode, with repetition rates up to about 200 kilohertz (kHz). Power control is effected by taking a small portion (about 1% or less) of the output power, delivering that to a detector of some kind, amplifying a voltage output signal of the detector and using the amplified signal to adjust the RFPS to stabilize the output power at a desired value.
The effectiveness of this output power stabilization depends strongly on the detector used and the amplifier or amplifiers used to amplify the voltage output signal of the detector. A particular problem is that an amplifier (or amplifier stage) typically exhibits a characteristic offset-voltage that is amplified together with the input voltage to the amplifier.
For some sensors, such as thermocouples or photodetectors for example, extremely high amplifier gains (on the order of 104 to 106 or greater) may be required to produce a signal that is usable by subsequent signal-processing stages. At such high gains the offset-voltage of the amplifier alone may result in amplifier output voltages that either saturate or severely limit the dynamic range of the amplifier.
By way of example a representative precision amplifier from one manufacturer has an offset-voltage of 10 microvolts (μV) minimum and 125 μV maximum. Accordingly, for a total gain of 25,000 the offset-voltage alone results in unwanted variations ranging from about 0.25 V to over 3 V. With the trend to lower and lower supply voltages these output levels represent a serious limitation. This limitation is further compounded by drift of the offset-voltage that can be introduced by changes in temperature or device aging.
In order to address this limitation, IC amplifier suppliers have developed amplifiers that use active techniques to reduce both amplifier offset-voltage and drift. These amplifiers are usually termed auto-zero or chopper-stabilized amplifiers and employ various techniques to measure and remove the offset-voltage component from the amplifier's output signal. While the chopper-stabilized and the auto-zero amplifiers have notable differences both rely on switching techniques to achieve desired results. These switching techniques degrade both the noise performance and useful bandwidth of the amplifier.
FIG. 1 schematically illustrates a simplified prior-art arrangement 10 for controlling the average output power of a laser 12 driven by an RFPS 20. A small sample, for example about 1%, of the output beam of the laser is reflected by a low-reflecting mirror 14 and directed onto a photodetector 16, while major portion of the laser beam is propagated to the work piece. The detector provides a voltage output signal which varies in proportion to the reflected sample and, accordingly, in proportion to the output power of the laser. The signal output from detector 16 is connected to an electronic controller 18. A user sets a desired laser output power and other operational parameters via command signals provided to the controller. After appropriately scaling the signal from the detector, the controller compares the scaled signal to the power specified by the user. If the power is not as specified, the controller sends signals to the RFPS 20 to appropriately adjust the RF power delivered to the CO2 laser discharge to maintain the laser output power at the level specified by the user.
In early infrared (IR) laser systems, detector 16 was a common thermopile or pyroelectric type IR detector. Due to certain limitations of these IR detectors, they found only limited use in commercial lasers. A significant limitation of the thermopile detectors was a slow response time, typically range of about one second, or somewhat less, at room temperature. Pyroelectric detectors had a relatively fast response of about 1 microsecond (μs) but a low output compared with that of the thermopile detector.
Recent commercial availability of fast response-time, conductively cooled, thermoelectric IR sensors based on epitaxial grown thin films of high temperature superconducting compounds, such as YBa2Ca3O7 has renewed interest in active CO2 laser output power control systems based on arrangement of FIG. 1. Such detectors have a response time that is comparable to that of a pyroelectric detector but having a DC output response comparable with that of the thermopile detector, albeit still relatively low. The faster response time enables wide-band control of the average laser output power even up to a pulse by pulse control. The detectors have an ability to handle high average power (on the order of tens of watts) without optical damage.
FIG. 2 schematically illustrates one prior-art arrangement of controller 18 in arrangement 10 of FIG. 1. In this arrangement detector 16 is assumed to be a thermo-electric IR detector of the type discussed above. Here, the controller includes an integrated circuit pre-amplifier (PA) 22, an analog-to-digital converter (A/D) 24, power control programmable logic 26, and pulse width modulation (PWM) circuitry 28. The signal from detector 16 is connected to pre-amplifier (PA) 22 the output of which is connected to A/D converter 24. The digitized output is delivered to power control logic 26 which is provided with digital commands (from a PC or the like) including a specified output power. The power control logic delivers digital signals to the PWM circuitry. This PWM circuitry delivers a pulse train to RFPS 20. The duty cycle (pulse duration divided by the pulse repetition period) sets the average power delivered by the RFPS. Typically the user sets the desired average laser power and pulse repetition rate and the PCL varies the duty cycle to maintain the laser output at the level specified by the user. Typical pulse repetition rates are in the range of 10 kHz to 200 kHz, with duty cycles ranging from 20% to 60%. This form of closed-loop control by pulse-width modulation is well known in the art and broadly applied in a variety of applications. Typically, the power control is effected by periodically measuring (with the control electronics) the output power during a time period when the laser is performing an application and correspondingly adjusting or not adjusting the RFPS output to stabilize the output power at the desired level. The measurement period is determined, inter alia, by parameters of the electronic control loop.
As noted above, there is a downward trend in the supply voltages used in modern integrated circuit amplifiers and reference voltages of modern A/D converters. This is driven by a variety of factors, but an end result is that typical integrated circuit amplifiers operate from a total supply voltage of 5 V or less. This limits the allowable A/D reference voltages to typical values of 4.096 V. Because of this, the A/D output scale-factors can range from 4 mV/W for a 1000 W laser to 40 mV/W for a 100 W laser. Due to the relatively low sensitivity of a thermo-electric detector the preamplifier gain required to provide a usable signal to the A/D converter 24 needs to be high, for example, on the order of 10,000. Such high gain results in large DC errors due to the amplification of the offset-voltage of the pre-amplifier.
Further as noted above, so-called auto-zero and chopper-stabilization techniques have been developed to attempt to deal with the problem of offset amplification, but these result in a high output-noise. This makes it necessary to severely limit the pre-amplifier bandwidth, and ultimately the control loop bandwidth, in order to maintain the necessary closed loop power stability. This bandwidth limitation results in laser material processing system throughputs that are far below the capabilities of present day technologies in high speed scanning mirrors, fast response IR detectors, and pulse performance of high power lasers. There is a need to find a solution for compensating the offset-voltage amplification that does not have significant noise as a by-product.